How the price to earnings ratio works

A price to earnings ratio of 20 means buyers are paying twenty times one year of profit for the shares, or, read the other way, that current profit would take twenty years to add up to the price.

$$\text{P/E} = \frac{P}{\text{EPS}} = \frac{\text{Market cap}}{\text{Earnings}}$$

On a $50 share with $2.50 of earnings, P/E is 20. With 100,000,000 shares outstanding, market capitalisation is $5,000,000,000 and total earnings are $250,000,000. Divide those two and the ratio is 20 again.

The two routes are the same identity. Multiplying price and EPS by the share count cancels, so market cap over total earnings cannot disagree with price over EPS unless one of the three inputs is from a different date than the others.

The price to earnings calculator on this page works both routes from one set of figures.

Keep the $50 price and 100,000,000 shares. Raise EPS to $5.00. P/E falls to 10. Market cap is still $5,000,000,000. Total earnings are now $500,000,000.

The price did not move. The denominator did. A lower P/E here is a more profitable year, not a cheaper share. That is why a low P/E at the top of a cycle, when earnings are at a peak that will not repeat, can be the expensive reading: the market is already looking through the peak, and the ratio is being divided by a number that is about to shrink.

The opposite error is reading a high P/E as a bubble. A high multiple can be growth that has not arrived in the trailing figure yet. Forward P/E tries to put that growth in the denominator. It then moves whenever the estimates do, which is a different object from a trailing P/E of 20 on reported earnings of $2.50.

Type the EPS your sheet is using. The formula does not know which one you meant.

Price is now $80, EPS still $2.50, shares 50,000,000. P/E is 32. Market cap is $4,000,000,000. Total earnings are $125,000,000.

The ratio is higher than 20 and the firm is smaller than the first sheet. P/E hides scale. It also means most inside a sector: a regulated utility with flat earnings and a software firm reinvesting everything are not priced on the same scale, and neither are markets in different countries.

Enterprise value over EBITDA is a different ratio on a different claim. Debt sits in the EV numerator and the denominator is before interest. Do not line a P/E up next to EV/EBITDA and call the gap a finding. One prices the equity residual after interest. The other prices the operations before it.

A discounted cash flow is the model that tries to say what the share is worth from the cash the operations will produce. P/E is a snapshot of what buyers are paying for this year's profit. The two answers can sit far apart, and that gap is information rather than an error in either method.

When earnings are zero or negative the ratio stops working. Dividing by a loss does not produce a useful multiple. This calculator prints no P/E in that case rather than a nonsense figure. Loss-making firms get compared on revenue, or on a different multiple, not here.

Two versions circulate and they are not interchangeable. Trailing P/E uses the last twelve months of reported earnings, so it is factual and backward-looking. Forward P/E uses estimates for the year ahead, so it is a forecast wearing a ratio's clothes. Mixing them in a table is how a cheap-looking name appears next to an expensive-looking one that is actually the same firm on two different denominators.

The four families on financial ratios explained are built from the accounts alone. P/E needs a share price as well, so it sits outside those four on purpose. The rule about comparables still applies: read it against the same firm over time, or against a rival doing the same work.

This page is the one division, the two routes that have to meet, and the mistake of reading a move in the ratio as a move in the price. It is not a trailing-against-forward switch, not a sector screen, and not a buy or sell. The figures throughout are a teaching sheet: a $50 share, $2.50 of earnings, 100,000,000 shares, then the same price on $5.00 of earnings, then an $80 share on 50,000,000 shares. This is educational material, not financial advice.